The objective of this research is to establish in detail the physiological properties of inputs from the basal forebrain to the limbic system in adult rats. Three specific pathways will be studies: (1) projections from the nucleus of the diagonal band of Broca to the posterior cingulate cortex, (2) projections from the diagonal band to the entorhinal cortex, and (3) projections from the ventral pallidum/nucleus basalis to the basolateral nucleus of the amgydala. The pathways will be studied by the neurophysiological techniques of in vivo intracellular recording and labeling. The basal forebrain will be activated by electrical stimulation, and neuronal responses will be recorded intracellularly in the target areas. Target neurons in the cingulate and entorhinal cortices and the amygdala will be morphologically identified by labeling them with intracellular injections of horseradish peroxidase (HRP). The basal forebrain modulation of synaptic transmission in thalamo-cingulate projections will be studied. A final goal is to inject basal forebrain neurons intracellularly with HRP in order to characterize in detail their pattern of axonal projections and coilateralization. The studies will establish the in vivo physiological and morphological relationship between the extensive projection system of the basal forebrain, and three important components of the limbic system, the cingulate cortex (one of the important cortical targets of the hippocampal formation), the entorhinal cortex (a principal source of input to the dentate gyrus and hippocampus), and the amygdala. These intracellular studies will thus fill significant gaps in our knowledge about the synaptic organization and physiology of these systems.